1. Field of the Invention
The invention relates to a centrally controlled telephone exchange, more specifically a private branch exchange comprising a plurality of subscriber terminal stations and directional data lines, to which all the subscriber terminal stations are connected in parallel and via which data blocks are serially transmitted.
2. Description of the Related Art
WO No. 80/00883 disclose a data system controlled in accordance with the time-division multiplex principle. This data system comprises a plurality of data processing units connected to a central control. The data processing units each comprise a transmitter and a receiver. Each transmitter and each receiver are connected to a single bus line. Amplifiers can be provided in each of these bus lines (directional data line), as a result of which very long bus lines can be employed.
Via one bus line data blocks are serially transmitted from a transmitting data processing unit to the central control. After processing in the central control the information to be exchanged between the data processing units is transmitted in the form of serial data blocks via the second bus line to the addressed data processing unit.
In this data system (WO No. 80/00883=P No. 29 53 239.6) control signals and information (data) are jointly transmitted via a bus line. The closed loop between two information-exchanging data processing units is always conveyed via the central control. For the determination of the conditions as regards time for the control the occurrence of longer time delays caused by long bus lines and a response time determined by a fixed instruction programme in the central control must be taken into account. This predetermined, unchanging overall time delay and the dead time determined thereby reduces the total capacity of the duplex transmission channel formed by the two bus lines and renders an undelayed flow of information impossible.
If, for example analog speech signals (information) are converted into digital signals in subscriber terminal stations and these digital signals are transmitted in such a data system, the delay caused by the processing operation can be long. This delay may become manifest as an echo, that is to say as interference noise. A further disadvantage is that when central control fails, for example in the event of an interruption in one of the two lines no communication is possible any more between the data processing units. This failure results in failure of the whole data system. The probability that this occurs, is called the failure probability.